Generally, the known pneumatic door closers include an elongated cylinder having a piston connected to a piston rod reciprocally displaceable within the cylinder, the piston and associated piston rod being normally biased by a spring toward the closed position of the door. With the free end of the piston rod extending beyond an end wall of the cylinder and the other end of the cylinder suitably connected between a door frame and the door, the opening of the door causes the piston to be rectilinearly displaced within the cylinder, whereby the spring is forcibly biased by the displacement of the piston within the cylinder as the piston rod is extended through the associated end wall of the cylinder. The force of the spring bias acting on the piston normally functions to return the door to the closed position as the door is released after the opening thereof. During the door closing motion of such known pneumatic door closers, the displaceable piston, under the bias of the spring, tends to compress the air medium in the chamber of the cylinder opposed to the piston rod so that the resultant air pressure acting on the piston tends to slow down the piston and thus the closing of the door.
However, it has been noted that a major inconvenience or disadvantage with such known pneumatic door closers is that after an associated opened door has been released for closing, the door initially closes at a relatively rapid closing speed for about two-thirds of the closing stroke of the piston or until such time that the air within the pressure chamber opposed to the piston rod has been sufficiently compressed to a level that can counterbalance the force of the spring and the inertia of the door so as to exert a sufficient amount of pressure on the piston to control or slow the rate of closing of the door during the final one third phase of the closing stroke so as to avoid any damage or banging of the door or injury to the user. It has been observed that such pneumatic door closers are able to achieve a reasonable controlled rate of closing only during the approximately last one third of the closing stroke of the piston. From a user's point of view, a door equipped with such known pneumatic door closers is not user friendly, as the initial high closing speed over the initial two thirds of the closing stroke can cause injury to an unsuspecting user and/or unnecessary damage or excessive banging of the door on closing.
While the foregoing noted problems may be avoided by utilizing hydraulic door closers which are able to achieve a more uniform or controlled closing speed due to the incompressibility of a liquid medium, such is not a practical solution as hydraulic closers are relatively more expensive and are also subject to other problems in the event of any loss or leakage of the hydraulic fluid.